Thin film flakes such as color-shifting flakes have been manufactured for decades using what is now a standard process. A supporting web or substrate, typically polyester terephthalate (PET) is coated with a thin film optical coating having a desired optical effect. However, before applying the thin film optical coating to the PET a very thin release layer of soluble organic or inorganic material such as NaCl, wax, silicon or other suitable material is applied to the PET that allows the thin film optical coating coated thereover to be removed from the PET supporting layer. Generally the thin film coating is released from the PET by passing it through a stripper containing solvent such as acetone or in some instances water, then over rollers and in a chamber containing solvent. In order to lessen the amount of solvent required to dissolve the release layer, and to reduce materials that have to be used in this process, it is desired to have the release layer be as thin as possible. In practice, the thickness of the release layer is many orders of magnitude thinner than the substrate it is coated on.
In the process of manufacturing thin film interference optical flakes the thin film optical coating is often applied by coating an absorber layer on top of the thin release layer in a vacuum chamber and subsequently coating a dielectric spacer layer over the absorber layer and then coating a reflective layer over the dielectric spacer layer.
This process is described by Roger Phillips in U.S. Pat. No. 4,705,300, by Coulter et al. in U.S. Pat. No. 6,383,638 and by Argoitia in U.S. Pat. No. 7,258,915 all of which are incorporated herein by reference, for all purposes.
U.S. Pat. No. 6,235,105 in the name of Hubbard discloses multilayer thin film materials produced by pre-coating a carrier substrate with a soluble release coating and depositing thin films of the optical materials in preselected sequence onto the carrier to build up the desired multilayer thin film structure, similar to the steps described above. Hubbard also discloses that alternatively, the substrate itself may be soluble. After deposition of the multilayer structure, the release coat (or the substrate) is dissolved, thereby freeing the multilayer structure from the substrate so that the same can be broken up into small flakes.
In the past number of years, there has been increased interest in providing thin film flakes that have indicia stamped therein. Flakes with Euro symbols or specialized symbols have been used on numerous documents and tokens requiring security features to verify authenticity. For example, casino tokens have been coated with thin film coatings that include flakes with particular stamped indicia.
U.S. Pat. No. 7,645,510 in the name of Argoitia discloses flakes having grooved frames and symbols thereon.
The process of stamping or embossing an embossable coated layer is disclosed U.S. Pat. No. 6,468,380 in the name of Miekka et al. incorporated herein by reference. Miekka et al. describe in detail a method of embossing a coated sheet with a microstructure in the form of a diffraction or holographic pattern. Miekka, in a similar manner as embossed thermoplastic layers are used today, utilizes a substrate of heat resistant polyethylene terephthalate (PET) base layer. The heat resistant PET is typically coated with a thermoplastic coating such as polyvinylchloride. An embossing pattern in holographic form can be formed on the thermoplastic layer which desirably contains a release agent such as hydroxylated polysiloxane which allows separation of the embossing from the embossing roller.
Miekka further teaches as a first step of fabricating a grating or hologram, the provision of a heat resistant plastic film with a thermoplastic coating that has a softening temperature below the heat-distortion temperature of the heat resistant film. The thermoplastic is then heated to above its softening temperature and the softened thermoplastic layer is contacted with an embossing master. The contacting step involves the contact of the thermoplastic layer with an embossing master, which is an embossing roller, a metallic die, or a metalized plastic film, on the roller. The master can be used to form a holographic or diffractive pattern in the thermoplastic layer. The thermoplastic surface is heated to above its softening temperature; and the softened thermoplastic surface is contacted with an embossing master at temperatures below the thermoplastic softening temperature of the coating so that rapid embossing takes place and reflow of the plastic is avoided after separation of the coating.
As of late, in an attempt to fabricate multilayer flakes with a predetermined pattern or microstructure thereon or therein, a separate thin release layer is coated over top of the embossed thermoplastic layer.
The thin release layer can be NaCl, Na2B4O7 hydrated (Borax) or another type of release layer is coated upon the stamped material which is subsequently coated in a vacuum chamber. Since the NaCl and borax are soluble in water, the coated product is then placed into a container of water and the coating is removed.
In some instances we have found that release layers such as NaCl or borax, left on the coating are harmful to one or more layers, for example to the absorber layer on color shifting flakes. Furthermore, it is desirous not to provide this thin release layer as its application includes an additional production step in manufacturing and extra materials being used.
The typical process for producing thin-film flakes having microstructures therein has been to provide a substrate with a high softening temperature, coated or laminated with an embossing material having a lower softening temperature, then stamping the embossing material under suitable heat and pressure so as to form an embossing, and subsequently coating the embossed material with the thin release layer (NaCl or Borax) described above, followed by coating the release layer with the thin film coating. The next step involves submersing the coated structure in a bath of water and wiping the structure so as to release the flakes.
In an effort to lessen the steps and materials used, and to produce a superior product this invention provides a solution that eliminates the addition of a release layer over the stamped embossed layer.
It is a further object of this invention to fabricate thin film flakes having a microstructure therein, wherein the flakes are absent unwanted residue often associated with release layers such as NaCl or borax.
It is an object of this invention to provide a method of fabricating flakes wherein a dissolvable coated microstructure obviates the use of a very thin standard release layer.